1. Field of the Invention
The present invention relates to a radio communication apparatus, radio communication method and program storage medium, and, for example, to a radio transmission technique using a plurality of antennas.
2. Related Art
There is known a spatial division multiple access (SDMA) scheme which is a technique of multiplexing a plurality of users at the same frequency and the same time using a plurality of antennas.
Now, suppose a base station (radio communication apparatus) has two antennas (antennas 1 and 2) and two user terminals (user terminal 1, user terminal 2) each have one antenna.
Suppose transmission signals to the user terminals 1 and 2 ares=[s1,s2]T  [Formula 1]and noise signals of the user terminals 1 and 2 aren=[n1,n2]T  [Formula 2]received signals of the user terminals 1 and 2 can be described as
                                                        y              =                              Hs                +                n                                                                                        =                                                                    [                                                                                                                        h                            11                                                                                                                                h                            12                                                                                                                                                                            h                            21                                                                                                                                h                            22                                                                                                                ]                                    ⁡                                      [                                                                                                                        s                            1                                                                                                                                                                            s                            2                                                                                                                ]                                                  +                                  [                                                                                                              n                          1                                                                                                                                                              n                          2                                                                                                      ]                                                                                        [                  Formula          ⁢                                          ⁢          3                ]            
Here, “H” is a channel matrix, “h11” denotes a channel response from the antenna 1 to the antenna of the user terminal 1, “h12” denotes a channel response from the antenna 2 to the antenna of the user terminal 1, “h21” denotes a channel response from the antenna 1 to the antenna of the user terminal 2, and “h22” denotes a channel response from the antenna 2 to the antenna of the user terminal 2.
Here, when the base station multiplies the transmission signal “s” by a weight:W=H−1  [Formula 4]and transmits the signal, received signals of the user terminals 1 and 2 become
                                                                        y                ′                            =                              HWs                +                n                                                                                        =                              s                +                n                                                                                        =                                                [                                                                                                              s                          1                                                                                                                                                              s                          2                                                                                                      ]                                +                                  [                                                                                                              n                          1                                                                                                                                                              n                          2                                                                                                      ]                                                                                        [                  Formula          ⁢                                          ⁢          5                ]            That is, the user terminals 1 and 2 can receive only the signals “s1” and “s2” without mutually causing interference. This realizes SDMA and suppose this SDMA is particularly called “ZF (Zero Forcing)-SDMA.”
However, since ZF-SDMA uses such weights that cause mutual interference to become 0, the characteristic of each user terminal is the same characteristic as that in the case with reception using one antenna, resulting in a problem that suitable performance cannot be obtained.